This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Objectives: To (1) determine whether sequence variants within gp120 are selected during transmission;(2) determine whether selection of viral variants reflects the population of virus present in the genital fluids of the donor or a biological restriction of the transmission process;(3) determine the frequency, kinetics, and the virologic and immunologic ramifications of HIV superinfection in both partners following early infection;and (4) to determine if uninfected partners sexually exposed to virus produce protective antibodies in their genital tract, thereby reducing opportunity for productive infection. Heterosexual HIV transmission is primarily a monophyletic event. To probe whether limited genetic diversity in the genital tract of the transmitting partner explains this bottleneck, viral envelope sequences from blood and genital fluids of 9 transmission pairs from Rwanda and Zambia were analyzed. The transmitting partner's viral population was heterogeneous with distinct stable genital sub-populations. Surprisingly, the transmitted founder variant was not derived from the predominant genital tract sub-populations. Hence, the transmission bottleneck is not driven by limited viral diversity in the genital tract, but instead involves selection of a more transmissible variant from the donor genital tract. Thus far, 4/44 individuals were also identified as being superinfected, but screening is costly and slow. 454 sequencing is now being used to enhance screening throughput. Genital secretions are being collected from seronegative exposed females and infected positive controls to determine whether local immunoglobulins in the genital tract protect against viral transmission and studies will also investigate whether protection involves CD8+ T lymphocytes.